1. Field of Use
This invention relates generally to electric or electronic intrusion or burglar alarm systems, and particularly to systems wherein actuation of a switch by opening a door or window to gain access therethrough causes actuation of an alarm for an interval of time.
2. Description of the Prior Art
Some prior art electric or electronic intrusion or burglar alarm systems are designed to warn of and discourage attempts by an unauthorized person to enter a structure, such as a building or vehicle, by opening a door or window to gain access therethrough. Such prior art systems typically comprise a switch actuatable by the door or window (hereinafter simply referred to as "door"), a control circuit operatively connected to the switch and responsive to switch actuation, and an alarm, such as a bell or siren, connected to be operated by the control system. In operation, opening of the door actuates the switch thereby causing the control system to operate the alarm with the two-fold object of summoning a responsible person, such as a householder or guard, and of frightening the intruder away. Some control systems operate the alarm until appropriate personnel are summoned and operate a shutoff switch, whereas other control systems embody means to shut off the alarm automatically after an appropriate interval of time, usually several minutes. The following U.S. Patents illustrate the state of the art: U.S. Pat. Nos. 4,188,621, 4,174,516, 4,164,736, 4,156,235, 4,137,526, 4,074,248 and 3,626,403.
In some cases, an intruder knowledgeable in the operation of prior art intrusion or burglar alarm systems, will postpone entry through the open door until the alarm automatically shuts off and, if no responsible person has appeared by then, will enter the structure, closing the door after himself to create the appearance of normality. The possibility of a responsible person failing to respond to the alarm is increased in remote localities where few people are about or, sometimes, in localities where high background noise occasionally overcomes or obscures the sound of the alarm.